The effect of neonatal blockage of the pituitary-testicular axis with gonadotropin releasing hormone (GnRH) antagonist on the tempo of sexual maturation and immune system development is being assessed in male monkeys. The disruption of normal neonatal activity of this axis was associated with retarded sexual development, but social rank was the key factor in determining the timing of puberty in male monkeys. The effect of neonatal treatment with a GnRH antagonist and low rank could not be reversed by neonatal exposure to greater than normal levels of androgen. GnRH antagonist treatment also impaired development of immune function. Treatment animals exhibited abnormal lymphocyte profiles that persisted through early adulthood, and impaired cell- and humoral-mediated immune response to foreign antigens. The results suggest that the neonatal period of male primates is a critical interval in development, and that environmental or endocrine perturbations during this period may profoundly alter reproductive development and immune function for many years to come.